1. Field of the Invention
The present invention relates to an airtight cap structure having a cap detachably covering a tip of an instrument such as stationeries and cosmetics for preventing the tip from drying.
2. Description of the Related Art
A conventional cap structure is described e.g. in Japanese Patent Laid-Open No. H9-30187. The cap structure described in Japanese Patent Laid-Open No. H9-30187 is a cap structure for a low-viscous ink writing instrument, wherein a cap comprises an outer cap and a cup-like sealing element fixed inside the outer cap and made of an elastic material, and a free peripheral edge of the sealing element comes into close contact with a tip periphery of an axial sleeve when the cap is mounted.